Christer Björkman

Effects of nitrogen fertilization on pine needle chemistry and sawfly performance

SummaryChanges in needle nitrogen and resin acid concentrations in young Scots pine trees fertilized with ammonium nitrate were followed over 3 years. Sawfly larvae (Neodiprion sertifer) were reared on fertilized and control trees the year after fertilization. Both nitrogen and resin acid concentrations increased in fertilized trees. The fact that resin acid concentrations increased contradicts predictions of the carbon/nutrient balance hypothesis. We suggest that needle resin-acid concentrations are limited more by the size of the resin ducts than by the availability of substrate for resin acid synthesis, and that the formation of resin ducts is limited by the availability of nitrogen. A modification of the carbon/nutrient balance hypothesis, relating compartment formation to allelochemical synthesis, is discussed. Performance of sawfly larvae was not affected by fertilization treatment, probably because concentrations of nitrogen (positively affecting performance) and resin acids (adversely affecting performance) increased simultaneously in fertilized trees. Thus, the results of this study do not support the notion that fertilization increases the resistance of trees to needle-eating insects.

Responses of Neodiprion sertifer (Hym., Diprionidae) larvae to variation in needle resin acid concentration in Scots pine

SummaryWe have studied how differences in needle resin acid concentrations between two clones of Scots pine influenced larval survival, larval developmental time, and cocoon weight in Neodiprion sertifer. Larvae were reared under controlled conditions in the laboratory on needles showing a three-fold difference, in resin acid concentration. Larval developmental time was significantly longer for larvae fed needles high in resin acids. No significant differences were found in cocoon weight. Larvae fed needles high in resin acids suffered significantly higher mortality, most of which occurred, in the first two instars. Data on relative growth rate for last-instar larvae, based on a traditional growth efficiency experiment, did not show any negative effects of resin acids, indicating that a possible induction takes place during early instars. Observations on the feeding behaviour of last-instar larvae, the high consumption of bark tissue (with high concentrations of resin acids) by larvae fed needles low in resin acids and preference for needle tissues high in resin acids, indicate that these larvae might actively search for tissues rich in resin acids. We conclude that the physiology and behaviour of N. sertifer has been shaped, in part, by the conflict between the negative effects of resin acids, primarily in early instars (longer developmental time with a corresponding increase in exposure to natural enemies and an increase in direct mortality), and the positive effect of resin acids in later instars (a need for resin acids for use in its own predator defence secretion).

Oviposition preferences in pine sawflies : a trade-off between larval growth and defence against natural enemies

The oviposition preferences and performance of a monophagous pine sawfly (Neodiprion sertifer) were studied in the field on 61 naturally regenerated Scots pine trees. The preference of sawfly females, measured as the proportion of available shoots on a tree with egg batches, was found to be positively correlated with tree height and needle length in a multiple regression analysis. Females also seemed to prefer trees with high resin acid (diterpenoid) concentrations. Low variation among trees with respect to resin acid concentration, together with positive covariance between these concentrations and needle length, made it difficult to evaluate the role of resin acids. Even though pupae weighed less and larvae suffered higher mortality when protected from natural enemies on trees preferred by females, there were benefits associated with oviposition on such trees; i.e. uncaged larvae were less vulnerable to parasitoids on high resin acid trees. This indicates that females preferred trees on which their offspring gained enemy-free space over trees on which performance was high, which also was suggested by the weak interaction found between the effect of caging (enemy exposure) and oviposition preference on survival in early instars.

Pine sawfly defence and variation in host plant resin acids : a trade-off with growth

Abstract. 1 The defence capability of pine sawfly larvae reared on Scots pines with low or high concentrations of resin acids was determined in a field experiment, where larvae were exposed to predatory ants, and in a laboratory experiment in which the rate of defence droplet replenishment was measured. 2 Larvae on low resin‐acid shoots disappeared faster than larvae on high resin‐acid shoots when exposed to predatory ants. 3 Larvae fed high resin‐acid needles produced 50% larger defence droplets than larvae fed low resin‐acid needles. 4 In an additional experiment high predation pressure was simulated by repeatedly removing defence droplets. Larvae responded by feeding more on tissue rich in resin acids, but at the cost of a reduction in growth rate. 5 Based on these data and earlier findings that survival and development of sawfly larvae are negatively affected by high resin‐acid concentrations, we conclude that pine sawflies face a dilemma of conflicting demands, i.e. although high needle resin‐acid concentrations enhance defence capability, they also reduce growth and survival. Our data also suggest that it is not necessarily the ovipositing female that has to balance this trade‐off: plasticity in larval feeding behaviour provides larvae with a means to at least partly solve the dilemma.

Leaf Trichome Formation and Plant Resistance to Herbivory

arkk¨ ainen Leaf trichomes contribute to plant resistance against herbivory. In several plant species, the trichome density of new leaves increases after herbivore damage. Here we review the genetic basis of trichome production and the functional and adaptive significance of constitutive and induced trichome formation. We focus on leaf trichomes and their production in response to damage caused by herbivores. The genetic basis of trichome production has been explored in detail in the model species Arabidopsis thaliana. Recent comparative work indicates that the regulatory networks governing trichome development vary and that trichome production has evolved repeatedly among angiosperms. Induced trichome production has been related to increased levels of jasmonic acid in Arabidopsis, indicating a common link to other changes in resistance characteristics. Damage from insect herbivores is oftentimes negatively related to trichome production, and enhanced trichome production may thus be advantageous as it increases resistance against herbivores. There are yet few studies exploring the costs and benefits of induced trichome production in terms of plant fitness. Trichome density affects interactions with insect herbivores, but may also affect the abundance and effectiveness of predators and parasitoids feeding on herbivores, and the tolerance to abiotic stress. This suggests that an improved understanding of the functional and adaptive significance of induced trichome production requires field studies that consider the effects of trichome density on antagonistic interactions, tritrophic interactions, and plant fitness under contrasting abiotic conditions.

Adult beetle grazing induces willow trichome defence against subsequent larval feeding

Abstract. Induced increases in trichome density to grazing by herbivores have been suggested to increase plant resistance to future herbivore attack. In this paper we present results which show that plants of Salix cinerea L. respond to adult leaf beetle (Phratora vulgatissima L.) grazing by developing new leaves with an increased trichome density. The same plants are usually attacked again later in the season when the next generation of larvae hatches on the plants. The effect of the induced response was studied by comparing larval growth and feeding on newly developed leaves of plants exposed to different defoliation treatments. Larvae on plants previously exposed to adult grazing consumed less total leaf area and showed more dispersed feeding than larvae on plants protected from previous grazing. Larvae on plants exposed to mechanical defoliation responded intermediately. These results corresponded to the increased trichome density of defoliated plants. However, we found this larval response only in whole plant tests – when reared on single, excised leaves in petri dishes, larvae in all treatments behaved similarly. This discrepancy between the on-plant experiment and that in petri dishes highlights how experimental design may alter the conclusion of a study. We suggest that the induced response to adult grazing may act as a defence against subsequent larval feeding.

Effects of leaf beetle damage on stem wood production in coppicing willow.

1 The effect of defoliation by larvae of the leaf beetle Phratora vulgatissima on current‐year stem wood production of resprouting Salix viminalis was investigated for two years. Adjacent subplots with varying levels of defoliation within one large willow plantation in south Sweden were studied in the two years.

Ecological effects and management aspects of an exotic tree species: the case of lodgepole pine in Sweden.

The North American tree Pinus contorta var, latifolia was experimentally introduced in Sweden already in the 1920s, and has been used in Swedish forestry on a large scale since the 1970s. These pla ...

Leaf trichome responses to herbivory in willows: induction, relaxation and costs.

To circumvent the inherent problem of discriminating between the cost of losing photosynthetic tissue and the cost of producing an inducible defence, the growth response of herbivore-damaged plants was compared with plants damaged mechanically to the same extent but without eliciting the defence. Two experiments were conducted, studying the response of willows (Salix cinerea) to damage by adult leaf beetles (Phratora vulgatissima). In the first experiment, willows produced new leaves with an enhanced leaf trichome density 10-20 d after damage, coinciding in time with the feeding of beetle offspring. The response was relaxed in foliage produced 30-40 d after damage. In the second experiment, which also included mechanical damage, willows exposed to beetle feeding showed an increase in leaf trichome density of the same magnitude (> 70%) as in the first experiment. The cost of producing the defence was a 20% reduction in shoot length growth and biomass production. Willows exposed to mechanical damage had an 8% reduction in shoot length growth compared with control plants, that is, a cost of leaf area removal. The results are the first quantitative estimates of the cost of a plant defence induced by natural and low amounts (3.3%) of herbivory.

Harvesting disrupts biological control of herbivores in a short-rotation coppice system.

Disturbances such as harvesting often interfere with the ecological processes that lead to the biological control of insect pests. For willows, grown as short-rotation coppice crops harvested every third to fifth year, it has been suggested that high plant quality in the resprouting shoots after harvesting may explain observed high densities of herbivorous insects, especially leaf beetles (Coleoptera: Chrysomelidae), in the plantations. In this study, we show that generalist predators may be important as regulators of leaf beetle populations. All three leaf beetle species, which were studied for five years in 12 plantations, showed a negative correlation between the population growth rate from spring to fall and the abundance of the most common generalist predator Orthotylus marginalis (Heteroptera: Miridae). For the most abundant leaf beetle, Phratora vulgatissima, we also found a significant positive correlation between its population growth rate and egg survival, indicating an overall effect of predation on herbivore population growth. Harvesting, which takes place during the winter, had a negative effect on the abundance of leaf beetles and predators. However, the first year after harvesting, all three leaf beetle species regained this loss with a very high population growth rate. A reason for the better ability of the herbivores to recover from the disturbance may be that they, unlike the predators, mainly overwinter outside the plantations. All three leaf beetles peaked in density three years after harvesting whereas the density of generalist natural enemies increased or leveled off during the five-year period after harvesting. We conclude that predation by generalist predators is potentially important for population control of leaf beetles in willow coppice, but that the intermediate disturbance regime of around five years between harvests, appears to be too short to avoid disruption of biological control. Alternatives for more efficient biological control in short-rotation coppice systems may be a longer period between harvests that enables the predators to fully respond numerically, to leave natural enemies refuges at harvest, or to harvest adjacent plantations asynchronously.